Electric stimulus lights up carbon nanorings and iodine

8 August 2017

This is an illustration of the electric-stimuli-responsive porous carbon nanorings with iodine. An electric stimulus induces the hydrocarbon nanoring cycloparaphenylene-iodine assembly to become electrically conductive and emit white light.

Scientists at Nagoya University in Japan have developed a novel method for making stimuli-responsive materials, and used it to design a material made from a mixture of carbon nanorings and iodine that conducts electricity and emits white light when exposed to electricity. The team’s new method could help generate a range of reliable stimuli-responsive materials for use in memory devices, artificial muscles and drug delivery systems, among other applications.

Stimuli-responsive materials alter their own properties in response to external stimuli, such as photo-irradiation, heat, pressure and electricity. This feature can be controlled for a wide range of applications, such as in optical discs, computer memories and displays, as well as artificial muscles and drug delivery systems.

Researchers have been working to develop new stimuli-responsive materials in a predictable fashion. However, it has proved extremely difficult to design and control the complex molecular arrangements in these materials.

Now, a simple and reliable method to synthesize stimuli-responsive materials has been developed by a team led by Nagoya University’s JST-ERATO Itami Molecular Nanocarbon Project and Institute of Transformative Bio-Molecules (ITbM). The results of this study were recently reported in a paper in Angewandte Chemie.

This ‘responsive porous host’ method takes a molecule with a porous framework and incorporates a ‘guest’ molecule within it that is likely to react to external stimuli. In this case, the team found that [10]cycloparaphenylene ([10]CPP), a hydrocarbon molecule composed of 10 para-connected benzene rings, made an ideal host for iodine (I), which is incorporated inside the porous carbon rings.

Not only did the resultant material, [10]CPP-I, conduct electricity, but it also emitted a white light, which is unusual. Typically, many other components are required to emit white light. This shows the potential of the new material for next-generation illumination systems.

“This ‘responsive porous host’ approach is expected to be applicable to different stimuli, such as photo-irradiation, heat application and pH change, and open the path for devising a generic strategy for the development of stimuli-responsive materials in a controllable and predictable fashion,” said Hirotoshi Sakamoto, a group leader of the JST-ERATO project.

Synthesizing the material is surprisingly simple — the researchers mixed the carbon nanorings (CPP) and iodine together, and let it dry. X-ray crystallography confirmed that the iodine molecules line up inside the hollow cores of the aligned nanorings.

The team tried several variations of the mixture, changing the number of carbon nanorings, and found that 10 rings led to the most dynamic iodine atom movement and the most sensitive response to external environmental changes.

When a direct current was applied to [10]CPP-I, the bulk resistivity of the sample became approximately 380 times lower; the bulk resistivity in mixtures with nine or 12 nanorings did not decrease nearly as much. These results show that pore size in the nanoring assembly controls the response to electrical stimulation.

“One of the most difficult parts of this research was to investigate how the electric conductivity of [10]CPP-I is turned on by electric stimuli,” said Noriaki Ozaki, a postdoctoral researcher with the JST-ERATO project. “Although it only took us about three months to synthesize the molecule and discover its electric-stimuli-responsive properties, it took another year to discover the origin of its properties.”

The team finally figured out how the electric conductivity of [10]CPP-I is turned on by electric stimuli, using X-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy and fluorescence spectroscopy. These analyses showed that the iodine atoms in the carbon nanorings form extended polyiodide chains when stimulated by electricity, with these chains conferring electrical conductivity on the material.

The researchers also discovered that electric stimuli can switch the photoluminescence color of [10]CPP-I from a green-blue color to a white color. White luminescence means that the fluorescence spectrum of [10]CPP-I covers the whole visible light range. This spectral broadening is attributed to the irregular distribution of the electronic structures of the CPPs, which is caused by the formation of the polyiodide chains. It represents a rare example of white luminescence from a single molecular assembly; white light emission is usually achieved by mixing several components of different colors.

“We were really excited to develop this simple yet powerful method to achieve the synthesis of external-stimuli-response materials,” said Kenichiro Itami, director of the JST-ERATO project and center director of ITbM.

This story is adapted from material from Nagoya University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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